Sulfur isotope geochemistry of the Black Sea water column

We studied the isotopic composition of dissolved sulfide in the Black Sea water column during different seasons at a total of 15 stations spanning the entire basin. The isotopic composition of dissolved sulfide averaged over all depths varies between −42.0‰ and −32.6‰, ave. −39.6±1.3‰ (1σ) (118 data points). Seasonal and spatial (open sea vs. coastal stations) differences in the δ34S–H2S values are not observed. Slight 34S enrichments in the sulfide isotope composition are revealed in the uppermost and the lowest parts of the anoxic water column. The upper trend is explained as (i) the effect of mixing with 34S-enriched sulfide produced near the interface by chemical oxidation with MnO2 or O2, (ii) small fractionation during biological sulfide oxidation, (iii) a result of a decreased isotope fractionation factor due to higher sulfate reduction rates. The lower trend is likely the result of the mixing with 34S-enriched pore water sulfide. We generated the first isotope data for sulfur intermediates in the lower part of the anoxic zone, which show values close to the isotope composition of dissolved sulfide. We hypothesize that the high isotope depletions of sulfide observed in the entire Black Sea water column are a result of low sulfate reduction rates and superimposed disproportionation reactions within the oxidative part of the sulfur cycle. Different physical and chemical mechanisms facilitating the formation and transport of sulfur intermediates in the anoxic interior are discussed.